A bolt is a fastener that has a head and external threads made to fit with a nut. Industry standards require two types of marks on the head of the bolt. They are the name of the manufacturer and a grade mark that identifies the material properties. A raised mark on the head of a bolt indicates the strength and type of steel used during manufacturing. Bolts are graded by the type of steel used, size and pounds per square inch or strength. The higher the number, the higher the grade of material used in making the bolt, according to Nuts and Bolts: Determining Bolt Grades. Bolts that are not marked are inferior and do not meet minimum standards. Ask the store manager where the bolts were purchased to get the name of the manufacturer.
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Look at the head of the bolt for manufacturer's marks. The manufacturer makes a mark that can be either raised or depressed at their option. Reputable manufacturers stamp their trademarks in the head for easy identification and accountability. Compare this mark to a mark listed in a fastener identification guide book. The Northern Illinois University provides on its website the names of the three guide books available. Choose one to find where the bolt was made.
Rub your fingers lightly across the head of the bolt. You will feel raised numbers or marks. No lines on an unmarked bolt shows that the bolt was made from low carbon steel with 65,000 psi. This bolt can only be identified by asking the manager at the store where it was purchased. It does not meet industry standards. It would not pass construction standards but it may be fine for use in household repairs or outdoor furniture.
Feel for a circle inside a hexagon with the markings A-354-BD A490. This shows a bolt made from medium carbon alloy steel that has been quenched, heated steel submerged in oil or water to quickly cool it, and tempered to 150,000 psi. This is a bolt of good quality that can be traced to the manufacturer from an identification guide book.
Read or feel for the letters SAE, Society of Automotive Engineers, or the ASTM, American Society for Testing and Materials. These associations develop the standards for bolts used in the automobile industry, military, airline industry and many others. These markings tell you that the bolt was made by a company that is registered and accountable, thus listed in the Identification guide book. If the bolt is not marked with either SAE or ASTM it does not meet standards and will not be listed in the guide book.